The chances that anxiety disorder medication will help a patient is about as good as calling heads on a coin toss–and there is no way to determine who will benefit from medication and who will not.

K. Luan Phan, MD, University of Chicago, and colleagues, gave volunteers either a placebo or delta-9-tetrahydrocannabinol (THC)–the active ingredient in marijuana. While a participant’s brain was being scanned they were shown pictures of faces displaying a variety of emotions.

The subjects who received the placebo showed a response in the region of the brain called the amygdala, THC recipients had measurably less activity in the same region of the brain.